I posted about this in the PS+ free games thread, it is an excellent game - anyone who used to enjoy 1942, R-Type stuff like that, or more recently ResoGun should definitely have a go

I've been hammering it for the last week or so (haven't been on anything else), my 6 year old and my 4 year old can even play it on his own.

Very simple but before long there is absolutely loads going on on the screen and you can't progress through the levels without completing challenges (like kill 100% enemies, stay untouched, rescue all humans)

Yeah I noticed you on it, gets solid very quickly and you have to go back to earlier levels to complete the challenges, and the remaining ones are usually Destroy 100% of Enemies, and Stay Untouched, which are both nearly impossible after level 1!